One class of opto-electrical devices is that using an organic material for light emission or detection. The basic structure of these devices is a light emissive organic layer, for instance a film of a poly (p-phenylenevinylene) (“PPV”) or polyfluorene, sandwiched between a cathode for injecting negative charge carriers (electrons) and an anode for injecting positive charge carriers (holes) into the organic layer. The electrons and holes combine in the organic layer generating photons. In WO90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminium (“Alq3”). In a practical device one of the electrodes is transparent, to allow the photons to escape the device.
A typical organic light-emissive device (“OLED”) is fabricated on a glass or plastic substrate coated with a transparent anode such as indium-tin-oxide (“ITO”). A layer of a thin film of at least one electroluminescent organic material covers the first electrode. Finally, a cathode covers the layer of electroluminescent organic material. The cathode is typically a metal or alloy and may comprise a single layer, such as aluminium, or a plurality of layers such as calcium and aluminium.
In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic electroluminescent layer to form an exciton which then undergoes radiative decay to give light.
These devices have great potential for displays. However, there are several significant problems. One is to make the device efficient, particularly as measured by its external power efficiency and its external quantum efficiency. Another is to optimise (e.g. to reduce) the voltage at which peak efficiency is obtained. Another is to stabilise the voltage characteristics of the device over time. Another is to increase the lifetime of the device.
To this end, numerous modifications have been made to the basic device structure described above in order to solve one or more of these problems. Further layers may be provided between the electrodes and the organic light-emissive layer in order to aid charge injection and transport. It is particular preferred to use a hole injecting layer and/or a hole transporting layer between the anode and the light-emissive layer. The hole injecting layer may comprise a conductive polymer such as PEDOT:PSS. The hole transport layer may comprise a semiconductive polymer such as a copolymer of fluorene and triarylamine repeat units. The organic light-emissive layer may comprise a small molecule, a dendrimer or a polymer and may comprise phosphorescent moieties and/or fluorescent moieties.
The applicant's earlier published application, WO99/48160, discloses a light-emissive layer comprising a blend of materials including light emissive moieties, electron transport moieties and hole transport moieties. These may be provided in a single molecule or on separate molecules.
The use of a blend of materials in a single layer rather than providing separate layers of charge transporting and emissive material has the advantage that the materials can be deposited in a single pass, simplifying the manufacturing process for a device. Furthermore, the provision of a single layer having multiple functionalities may in principle result in better charge transport, charge transfer and emissive properties. Devices can also be made thinner due to a reduction in the number of layers which may reduce the voltage required to drive the device. Furthermore, a reduction in the number of interfaces between layers in a devices can be advantageous as interfaces can provide a source of structural defects in the device and may also be a source of detrimental electrical and optical effects such as impedance of charge flow through the device, light scattering and internal reflections.
While the use of blends in an opto-electrical device was proposed to have several advantages over the provision of multiple layers of material, it has been found that there are several problems with the use of blends in opto-electrical devices. It can be difficult to control the structure and properties of a blend. For example, the inter-mixing of materials in a blend can be difficult to control and may be unstable due to movement of chemical species within the blend, particularly when a device is driven. Provision of the different species in a single molecule can help to prevent differential movement of the species during driving of the device. However, multiple component polymers can be more difficult to make. Furthermore, even if the species are provided in the same molecule, partial phase separation of the species may still occur if the molecules change alignment during driving such that the species partially phase separate with like portions of the polymers forming domains within a layer. Such a change in the structure of a blend can result in a change in the functional properties of the blend during the lifetime of the device. As it is an aim to stabilise the voltage characteristics of an opto-electrical device over time, such an effect may be detrimental.
In light of the problems with using blends of material, many researchers in this field have looked at reverting back to the multi-layered device structure in which charge transporting and light-emissive components are provided in separate layers and considered how this arrangement could be improved.
In this regard, the applicant's earlier published application, WO 2006/043087, discloses cross-linking of polymers in opto-electrical devices in order to allow multiple layers to be deposited on top of each other without inter-mixing. This document discloses cross-linkable, hole transporting monomers which may be polymerised, deposited, and then cross-linked to form a hole transport layer on which an emissive layer can be deposited without dissolving the hole transport layer. It is also disclosed that the cross-linkable, hole transporting monomers can be mixed with light-emissive monomers, polymerised to form a polymer which has both the hole transporting monomers and the light-emissive monomers therein, deposited, and then cross-linked to form a light emissive layer on which an electron transporting layer can be deposited without dissolving the light-emissive layer.
The aforementioned arrangement gives a very robust device structure and allows multiple layers to be deposited without inter-mixing. However, it does not fully utilize the benefits of blending materials having different functionalities as discussed above in relation to WO99/48160.
WO 2005/049689 discloses cross-linkable fluorene compounds for use in electroluminescent devices.
Bozano et al, J. Appl. Phys. 2003, 94(5), 3061-3068 discloses cross-linked two-component blends for organic light-emitting devices.
As such, the present inventors have realized that it would be desirable to provide an arrangement in which the beneficial features of using a blend are present, but where the detrimental features are avoided.